(a) Field of the Invention
The present invention relates to a method of manufacturing a low melting-point alloy which is sealed in a fluorescent lamp, particularly, a low-pressure mercury vapor discharge lamp, so as to control the mercury vapor pressure therein, and to a fluorescent lamp having a low melting-point alloy manufactured by this method sealed therein.
(b) Description of the Prior Art
A low-pressure mercury discharge lamp such as a fluorescent lamp is known to have a highest efficiency of converting supplied electric energy into ultraviolet radiation of 253.7 nm wavelength when a mercury vapor pressure in a sealed tube is 6.times.10.sup.-3 to 7.times.10.sub.-3 mmHg and a discharge current is relatively low.
Since the ultraviolet radiation of 253.7 nm wavelength has a high fluorescent excitation efficiency, the mercury vapor pressure is preferably kept to fall within the above range of 6.times.10.sup.-3 to 7.times.10.sup.-3 mmHg and the temperature sealed tube wall at this time is preferably about 40.degree. C. However, recently developed low-pressure mercury vapor discharge lamps such as fluorescent lamps have higher loads acting on the sealed tube wall due to a tendency toward a smaller tube diameter. For this reason, the temperature of the sealed tube wall is high and exceeds 100.degree. C. in some cases.
When the sealed tube wall temperature becomes high, the mercury vapor pressure in the sealed tube exceeds 7.times.10.sup.-3 mmHg. Then, ultraviolet radiation components mainly having a wavelength of 253.7 nm are self-absorbed by mercury. This impairs the conversion ratio of the supplied energy into ultraviolet radiation and lowers the light output.
As a countermeasure against this, amalgam is conventionally sealed in the sealed tube so as to suppress an increase in the mercury vapor pressure at high temperatures. Japanese Patent Publications Nos. 54-33215 and 54-38582 describe fluorescent lamps in which an amalgam consisting of Hg, In, and one metal selected from Li, Al, Zn, Sn, Pb and Bi; or an amalgam consisting of Hg, Bi, and Pb or of Hg, Bi, Pb, and Sn.
Such an amalgam is sealed in the sealed tube by measuring a predetermined amount discharged from a vacuum suction thin tube having an inner diameter of about 2.0 to 2.5 mm. Therefore, conventionally, the amalgam is formed into particles by the atomizing method of spraying the amalgam in a molten state with a gas or by a method of pulverizing and granulating an amalgam ingot and measuring and sealing a predetermined amount of amalgam particles and sealing the measured amalgam particles in a sealed tube.
However, amalgam particles obtained by the atomizing method have a nonuniform shape and size and must be sieved by a sieve of a predetermined size for size adjustment before measurement and sealing into a sealed tube. This results in a very low yield and an expensive method. With the method of pulverizing the ingot, the resultant particles similarly have a nonuniform particle size and shape and also have cracks. Furthermore, since the central portion of the ingot has a higher content of Hg than anywhere else, the composition of the particles varies, resulting in a variation in a suppression effect of mercury vapor pressure upon sealing in a sealed tube.